Increase in superoxide production by heat-shocked cells of Neurospora crassa was demonstrated by a fluorometric assay. 2. A sensitive fluorometric assay for the estimation of superoxide anion radical--based on the liberation of 4-methylumbelliferone from 4-methyl-beta-D-umbelliferyl glucopyranoside--is described. 3. Using this system the level of superoxide in the medium of heat-shocked Neurospora crassa cells was found to be consistently higher, in comparison with that of non-shocked cells, cultured at the normal growth temperature of 28 degrees C. 4. Addition of superoxide dismutase to the culture media suppressed the production of 4-methylumbelliferone.
